Certain spinal conditions, including a fracture of a vertebra and a herniated disc, indicate treatment by spinal immobilization. Several methods of spinal immobilization are known, including surgical fusion and the attachment of pins and bone plates to the affected vertebras.
Spinal immobilization systems typically require the threaded securement of some form of bone anchor or bone screw-assembly into two or more vertebrae, which entails the drawing of the rod to the anchors/screw-assemblies, or drawing the anchors/screw-assemblies to the rod. Spinal screw-assemblies are used to secure a stabilization rod and comprise various components including a pedicle screw and a body member. The design of the spinal screw-assemblies allows for variable angular movement of the body member with respect to the pedicle screw with a threaded shaft portion of the screw extending through an opening in an end of the body member.
The next generation of pedicle screws is polyaxial screws, with a body member which pivots and rotates about the spherical head of a bone screw. The bone screw is captured in the body member with a bushing; the bushing in turn accepts a rod after the screw has been placed in the pedicle, and the rod is captured in the bushing/body member assembly by inserting a set screw into the threads of the body member. As with most polyaxial pedicle screws, tightening of the set screw applies pressure onto the rod, which translates pressure onto the bushing, which then applies pressure on the spherical head of the bone screw, locking the polyaxial motion. Unlike other polyaxial pedicle screws, the new pedicle screw achieves additional locking because of the design of the screw bushing; the lower portion of the bushing acts like a wedge so when force is applied, the bottom of the bushing wedges between the spherical head of the bone screw and the body member, providing additional locking force.
While this feature provides optimum performance with regards to strength of the construct in maintaining correction, it can make screw removal difficult, since the polyaxial motion remains locked due to the wedging effect, even after the set screw is removed. In order to restore polyaxial motion, the bushing must be un-wedged from between the bone screw head and the body member. The wedge can be knocked loose by tapping the body member of the bone screw with enough force to break the friction lock of the wedge, but this method may not be considered feasible in the case of patients with very poor bone.
As a result, a tool is needed which can interact with the bushing and pull up on it, restoring the bushing to its position prior to locking and thereby removing the wedge and restoring polyaxial motion.
Previously there has been no reliable method for restoring polyaxial motion to the screw. There has been no feature on the pedicle screw or bushing which allowed a tool to apply an upward force and remove the wedge. Tools have been attempted which rotate the bushing to “break” the wedge force, but these caused damage to the bushings and could not reliably effect the unlocking. The method of tapping on the screw body member to “break” the wedge has already been identified as one which cannot be recommended for patients with poor bone quality.